Multiscale Simulation
on a Light-Harvesting Molecular Triad
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Abstract
We have investigated the effect of solvation and confinement
on
an artificial photosynthetic material, carotenoid-porphyrin-C<sub>60</sub> molecular triad, by a multiscale approach and an enhanced
sampling technique. We have developed a combined approach of quantum
chemistry, statistical physics, and all-atomistic molecular dynamics
simulation to determine the partial atomic charges of the ground-state
triad. To fully explore the free energy landscape of triad, the replica
exchange method was applied to enhance the sampling efficiency of
the simulations. The confinement effects on the triad were modeled
by imposing three sizes of spherocylindrical nanocapsules. The triad
is structurally flexible under ambient conditions, and its conformation
distribution is manipulated by the choice of water models and confinement.
Two types of water models (SPC/E and TIP3P) are used for solvation.
When solvated by SPC/E water, whose HOH angle follows an ideal tetrahedron,
the structural characteristics of triad is compact in the bulk systems.
However, under a certain nanosized confinement that drastically disrupts
hydrogen bond networks in solvent, the triad favors an extended configuration.
By contrast, the triad solvated by TIP3P water shows a set of U-shaped
conformations in the confinement. We have shown that a slight structural
difference in the two water models with the same dipole moment can
have great distinction in water density, water orientation, and the
number of hydrogen bonds in the proximity of a large flexible compound
such as the triad. Subsequently, it has direct impact on the position
of the triad in a confinement as well as the distribution of conformations
at the interface of liquid and solid in a finite-size system